Belén Alemán

Strong Carbon Nanotube Fibers by Drawing Inspiration from Polymer Fiber Spinning

We present a method to spin highly oriented continuous fibers of adjustable carbon nanotube (CNT) type, with mechanical properties in the high-performance range. By lowering the concentration of nanotubes in the gas phase, through either reduction of the precursor feed rate or increase in carrier gas flow rate, the density of entanglements is reduced and the CNT aerogel can thus be drawn (up to 18 draw ratio) and wound at fast rates (>50 m/min). This is achieved without affecting the synthesis process, as demonstrated by Raman spectroscopy, and implies that the parameters controlling composition in terms of CNT diameter and number of layers are decoupled from those fixing CNT orientation. Applying polymer fiber wet-spinning principles then, strong CNT fibers (1 GPa/SG) are produced under dilute conditions and high draw ratios, corresponding to highly aligned fibers (from wide- and small-angle X-ray scattering). This is demonstrated for fibers either made up of predominantly single-wall CNTs (SWCNTs) or predominantly multiwall CNTs (MWCNTs), which surprisingly have very similar tensile properties. Finally, we show that postspin densification has no substantial effect on either alignment or properties (mechanical and electrical). These results demonstrate a route to control CNT assembly and reinforce their potential as a high-performance fiber.

Spatially Resolved Chemical Characterization with Scanning Photoemission Spectromicroscopy: Towards Near‐Ambient‐Pressure Experiments

The major experimental challenges in investigations of heterogeneous catalysis are the morphologically complex and dynamic micro‐ and nanosystems and the exploration of events that occur at the catalyst surface, which determine the catalyst activity and selectivity. Modern‐day investigations of catalytic reactions require a multitechnique experimental and computational approach, in which each tool provides specific and complementary information. The unique combination of surface and chemical sensitivity has ranked X‐ray photoelectron spectroscopy (XPS) as one of the most important experimental methods for the characterization of catalytic systems. After its invention, more than half a century ago, a revolutionary step in XPS development was the addition of sub‐micrometer lateral resolution intermediate between light microscopy and electron microscopy. XPS microscopes have responded to the increasing demands on nanotechnology to have access to the local chemical composition, electronic and magnetic structure, and reorganization processes at morphologically complex surfaces and interfaces. The high spatial resolution in XPS microscopes is achieved by two different approaches, magnification of the image of the irradiated surface area (X‐ray photoemission electron microscopy) or demagnification of the incident photon beam by using X‐ray focusing optics (scanning photoemission microscopy; SPEM). In the present article, using selected examples, we demonstrate the capabilities of SPEM for studies relevant to catalysis, and we will discuss the next steps in the ongoing development. In the first part, we present the successful characterization of the oxidation of (i) polycrystalline PtRh particles and (ii) Pd thin films that decorate carbon nanotubes. In the second part, we describe two new setups, developed at Elettra, to overcome the “pressure gap” for photoemission spectromicroscopy experiments, which is the major limitation in the exploration of “real world” catalytic reactions. The first measurements of core‐level photoemission spectroscopy and imaging obtained with spatial resolution of the order of 100 nm at near‐ambient pressure are presented.

Gas-phase functionalization of macroscopic carbon nanotube fiber assemblies: reaction control, electrochemical properties, and use for flexible supercapacitors

The assembly of aligned carbon nanotubes (CNTs) into fibers (CNTFs) is a convenient approach to exploit and apply the unique physico-chemical properties of CNTs in many fields. CNT functionalization has been extensively used for its implementation into composites and devices. However, CNTF functionalization is still in its infancy because of the challenges associated with preservation of CNTF morphology. Here, we report a thorough study of the gas-phase functionalization of CNTF assemblies using ozone which was generated in situ from a UV source. In contrast with liquid-based oxidation methods, this gas-phase approach preserves CNTF morphology, while notably increasing its hydrophilicity. The functionalized material is thoroughly characterized by Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, and scanning electron microscopy. Its newly acquired hydrophilicity enables CNTF electrochemical characterization in aqueous media, which was not possible for the pristine material. Through comparison of electrochemical measurements in aqueous electrolytes and ionic liquids, we decouple the effects of functionalization on pseudocapacitive reactions and quantum capacitance. The functionalized CNTF assembly is successfully used as an active material and a current collector in all-solid supercapacitor flexible devices with an ionic liquid-based polymer electrolyte.

Carbon nanotube fibers with martensite and austenite Fe residual catalyst: room temperature ferromagnetism and implications for CVD growth

We report on the room temperature ferromagnetic properties of continuous macroscopic fibers made up of carbon nanotubes grown by floating catalyst chemical vapor deposition. Their ferromagnetic behavior originates from the presence of residual catalyst nanoparticles: martensite with 0.77 wt% C content and FCC Fe. The first is intrinsically ferromagnetic, but the latter only due to severe lattice distortion as a consequence of C supersaturation. The stabilization of martensite and austenite occurs mainly because of the small diameter of the nanoparticles, in the range of 4–20 nm. This is smaller than the embryonic nucleus of the relevant equilibrium phases, but also implies that large C concentrations can build up in FCC Fe before C can be segregated as a stable graphitic nucleus. The room temperature remanence ranges from 10% to 25% and the coercivity from 55 to 300 Oe, depending on the choice of promoter for fiber synthesis (S or Se). Superparamagnetic behavior is only observed in S-grown samples on account of the smaller diameter of residual catalyst particles. The results of this work provide an explanation for the widespread observation of magnetic properties in oxide-free CNT samples produced by catalytic growth under a wide range of synthesis conditions.

Enhanced Electro-Fenton Mineralization of Acid Orange 7 Using a Carbon Nanotube Fiber-Based Cathode

A new cathodic material for electro-Fenton (EF) process was prepared based on a macroscopic fiber (CNTF) made of mm long carbon nanotubes directly spun from the gas phase by floating catalyst CVD, on a carbon fiber (CF) substrate. CNTF@CF electrode is a highly graphitic material combining a high surface area (~ 260 m2/g) with high electrical conductivity and electrochemical stability . One kind of azo dye, acid orange 7 (AO7), was used as model bio-refractory pollutant to be treated at CNTF@CF cathode in acidic aqueous medium (pH 3.0). The experimental results pointed out that AO7 and its organic intermediate compounds were totally mineralized by hydroxyl radical generated from Fenton reaction. In fact, 96.7 % of the initial TOC was eliminated in 8h of electrolysis by applying a current of -25 mA and ferrous ions as catalyst at concentration of 0.2 mM. At the same electrolysis time, only 23.7 % of TOC removal found on CF support which proved the high mineralization efficiency of new material thanks to CNTs deposition. The CNTF@CF cathode maintained stable its activity during five experimental cycles of EF set-up. The results indicated that CNTF@CF material could be a potential choice for wastewater treatment containing bio-refractory by electrochemical advanced oxidation processes (EAOPs).

Surface Chemistry Analysis of Carbon Nanotube Fibers by X-Ray Photoelectron Spectroscopy

Carbon nanotube fibers are materials with an exceptional combination of properties, including higher toughness than carbon fibers, electrical conductivity above metals, large specific surface area (250

Tensile properties of carbon nanotube fibres described by the fibrillar crystallite model

\frac{m^2}{g}

Controlling Carbon Nanotube Type in Macroscopic Fibers Synthesized by the Direct Spinning Process

) and high electrochemical stability. As such, they are a key component in various multifunctional structures combining augmented mechanical properties with efficient interfacial energy storage/transfer processes. This work presents a thorough XPS study of CNT fibers subjected to different purification and chemical treatments, including spatially-resolved micro XPS synchrotron measurements. The dominant feature is an inherently high degree of

Inherent predominance of high chiral angle metallic carbon nanotubes in continuous fibers grown from a molten catalyst

sp^2

Group 16 elements control the synthesis of continuous fibers of carbon nanotubes

conjugation, leading to a strong plasmonic band and a semi-metallic valence band lineshape. This high degree of CNT perfection in terms of longitudinal graphitization helps to explain reported bulk properties including the high electrical and thermal conductivity, and accessible quantum capacitance. There is also presence of organic impurities, mostly heavy carbonaceous molecules formed as by-products during fiber synthesis and which are adsorbed on the CNTs. Sulfur, a promoter used in the CNT growth reaction, is found both in these surface impurities and associated with the Fe catalyst. The observation of strongly-adsorbed surface impurities is consistent with the high ductility of CNT fibers, attributed to interfacial lubricity.